Chair base



FIG. 1 is a front perspective view of a chair base according to our new design;

FIG. 2 is a front/rear/left side/right side view thereof, it being understood that the front, rear, left side, and right side views are identical; and,

FIG. 3 is a top view thereof.

Features shown in dashed lines form no part of the claimed design. 

CLAIM The ornamental deign for a chair base, as shown and described. 